GNU Radio: Tools for Exploring the Radio Frequency Spectrum

Bringing the code as close to the antenna as possible is the goal of software radio. GNU Radio gives you the tools to join the communication revolution powered by today's fast processors.

Software radio is the technique of getting code
as close to the antenna as possible. It turns
radio hardware problems into software problems.
The fundamental characteristic of software radio is
that software defines the transmitted waveforms, and
software demodulates the received waveforms. This is
in contrast to most radios in which the processing is
done with either analog circuitry or analog circuitry
combined with digital chips. GNU Radio is a free
software toolkit for building software radios.

Software radio is a revolution in radio design due to its ability to
create radios that change on the fly, creating new choices for users.
At the baseline, software radios can do pretty much anything a
traditional radio can do. The exciting part is the flexibility that
software provides you. Instead of a bunch of fixed function gadgets,
in the next few years we'll see a move to universal communication
devices. Imagine a device that can morph into a cell phone and
get you connectivity using GPRS, 802.11 Wi-Fi, 802.16 WiMax, a
satellite hookup or the emerging standard of the day. You could
determine your location using GPS, GLONASS or both.

Perhaps most exciting of all is the potential to build
decentralized communication systems. If you look at today's systems,
the vast majority are infrastructure-based. Broadcast radio
and TV provide a one-way channel, are tightly regulated and the
content is controlled by a handful of organizations. Cell
phones are a great convenience, but the features your phone supports
are determined by the operator's interests, not yours.

A centralized system limits the rate of innovation. We could take
some lessons from the Internet and push the smarts out to the edges.
Instead of cell phones being second-class citizens, usable only if
infrastructure is in place and limited to the capabilities determined
worthwhile by the operator, we could build smarter devices. These
user-owned devices would generate the network.
They'd create a mesh among themselves, negotiate for backhaul and be
free to evolve new solutions, features and applications.

The Block Diagram

Figure 1 shows a typical block diagram for a software radio. To
understand the software part of the radio, we first need to understand
a bit about the associated hardware. Examining the receive path in
Figure 1, we see an antenna, a mysterious RF front end, an
analog-to-digital converter (ADC) and a bunch of code. The analog-to-digital
converter is the bridge between the physical world of continuous
analog signals and the world of discrete digital samples manipulated
by software.

Figure 1. Typical Software Radio Block Diagram

ADCs have two primary characteristics, sampling rate and
dynamic range. Sampling rate is the number of times per
second that the ADC measures the analog signal. Dynamic range refers
to the difference between the smallest and largest signal that can be
distinguished; it's a function of the number of bits in the ADC's
digital output and the design of the converter. For example,
an 8-bit converter at most can represent 256
(28) signal
levels, while a 16-bit converter represents up to 65,536 levels.
Generally speaking, device physics and cost impose trade-offs between
the sample rate and dynamic range.

Before we dive into the software, we need to talk about a bit of theory.
In 1927, a Swedish-born physicist and electrical
engineer named Harry Nyquist determined that to avoid
aliasing when converting from analog to digital, the ADC sampling
frequency must be at least twice the bandwidth of the signal of
interest. Aliasing is what makes the wagon wheels look like
they're going backward in the old westerns: the sampling rate of the
movie camera is not fast enough to represent the
position of the spokes unambiguously.

Assuming we're dealing with low pass signals—signals where
the bandwidth of interest goes from 0 to fMAX, the Nyquist
criterion states that our sampling frequency needs to be at least 2 *
fMAX. But if our ADC runs at 20MHz, how can we listen to
broadcast FM radio at 92.1MHz? The answer is the RF front
end.
The receive RF front end translates a range of frequencies appearing
at its input to a lower range at its output. For example, we could
imagine an RF front end that translated the signals occurring in the
90–100MHz range down to the 0–10MHz range.

Mostly, we can treat the RF front end as a black box with a single
control, the center of the input range that's to be translated. As a
concrete example, a cable modem tuner module that we've
employed successfully has the following characteristics. It translates a 6MHz
chunk of the spectrum centered between about 50MHz and 800MHz down to
an output range centered at 5.75MHz. The center frequency of the
output range is called the intermediate frequency, or IF.

In the simplest-thing-that-possibly-could-work category,
the RF front end may be eliminated altogether. One GNU Radio
experimenter has listened to AM and shortwave broadcasts
by connecting a 100-foot piece of wire directly to his 20M sample/sec
ADC.

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